The present invention relates to a control method whereby in automatic one-side welding the backing shoe is moved in synchronism with the welding electrode while adjusting the lateral position of the backing shoe with respect to the direction of welding.
An automatic one-side welding of the type in which the welding is perfomed while moving a welding electrode and a backing shoe along the welding seam of steel plates has been used widely in the welding of heavy steel plates such as shell plating assemblies of ships owing to its high efficiency. In order to weld together the plates having curved surfaces, it is essential that the travel speed of the welding electrode and the backing shoe respectively moving over the front and back surfaces of the steel plates along the curved surfaces thereof are synchronized with each other so as to always maintain them in proper alignment with each other. Thus a method is known in the art in which their travel speeds are preliminarily adjusted in accordance with the difference in curvature radius of movement between them. A disadvantage of this method is that since the calculation and setting of the travel speeds are extremely difficult in the case of welding hull blocks including curved portions with many different curvatures and since the calculation is made mainly on the basis of the design values, the deviation due to the error between the design values and the actual dimensions of the actual welding portions and the structural error of the transporting mechanism will be increased with an increase in the movement and the welding electrode and the backing shoe will be greatly shifted from their properly aligned positions, thus sometimes causing failure of the backing shoe to perform its function in extreme cases.
With the above type of automatic one-side welding method, measures are taken so that a transport supporting structure such as a rail is provided on each side of the steel plates to extend parallel thereto so as to move the welding apparatus and the backing shoe, whereby the backing shoe is displaced to follow the lateral displacement of the welding electrode with respect to the direction of welding and thus the backing shoe is positioned at the lateral center of the welding electrode with respect to the direction of welding. However, in the welding of such steel plates including a large number of curved portions with a variety of curvatures, to maintain the transport supporting structures provided on the sides of the steel plates in exactly parallel relation itself is not an easy matter and an error tends to occur in the synchronized displacements, thus causing the center of the backing material such as a backing glass fiber stacked tape to deviate from the position directly below the welding electrode and causing the burned area of the backing tape to extend to the tape edges and thereby tending to fail to produce the desired weld bead.
The method of moving a backing shoe in synchronism with a welding apparatus has been proposed, as for example, in Japanese Patent Publication No. 52-43612 in which ultrasonic sensors each detects arc sound at each of the front and rear ends of a backing shoe to control so that the arc position is always held within a predetermined area of the backing shoe. This method is disadvantageous in that the detection of the position of the arc point by means of sound waves causes external noise, mechanical vibrations, etc., to be superposed on the arc sound. Futher, the spacing between the sensors detecting arc sound cannot be selected wide enough due to the limited dimension of the backing shoe and therefore it is difficult to detect the difference in sound pressure level with a high degree of sensitivity. As a result, the accuracy of synchronizing the movement of the backing shoe with the movement of the arc position cannot be expected to be so high and there is not much possibility of using the detection signals for the purpose of controlling the welding parameters or the like, for example.